1. Field of the Invention:
This invention pertains to illumination devices and more particularly to emergency lighting circuits used in connection with gaseous-discharge lamps.
2. Description of Prior Art:
Gaseous-discharge lamps, such as mercury vapor and other metallic-additive vapor lamps, have long been employed in industrial lighting situations because of their extremely high efficiency when compared to other sources, such as incandescent lights.
A gaseous-discharge lamp is a source of radiant energy characterized by the emission of radiation from a stream of ionized vapor-carrying current between electrodes in the lamp. In starting the lamp, a relatively large voltage is required. However, once current flows in the lamp, the lamp exhibits a negative resistance characteristic. That is, the resistance of the lamp decreases with an increase in current. In common use in the United States are fluorescent, mercury and neon lamps.
To more fully discuss the operation of circuits of the present invention, it is first necessary to understand more fully the operation of these gaseous-discharge lamps. Typical of such lamps is the mercury-vapor lamp, which is used as an example for purposes of discussion.
The mercury-vapor lamp contains an arc tube filled with argon gas and a small amount of pure mercury. The arc tube is usually mounted within an outer bulb of glass. The arc tube itself it usually made of fused quartz. One main electrode extends into the tube from one end and a similar main electrode and a smaller starting electrode are at the other end of the tube. The starting electrode is electrically connected through a high resistance to the main electrode at the opposite end of the tube.
The mercury lamp is connected through its socket to the output leads of its ballast, which supplies proper voltage for starting and limits current during operation. When the ballast circuit is first energized, no current flows, and full starting voltage appears between the starting electrode and the adjacent main electrode. This voltage draws electrons across the relatively short gap, ionizing some of the argon gas in the tube and setting up a glow discharge between these two electrodes. The resistor in the circuit limits current to a few milliamperes. The ionized argon gradually diffuses through the tube, reducing the resistance in the gap between main electrodes. When resistance is low enough, an arc strikes across the main electrodes. The heat from the arc vaporizes the droplets of mercury, and they become ionized current-carriers as electrons in the arc bombard the vaporized mercury atoms. When all the mercury is vaporized, the current in the arc may reach several amperes. With this current flowing in the ballast, it no longer produces sufficient voltage to maintain the initial glow, which is extinguished. The arc is then maintained across the main electrodes with its current limited by the ballast.
If the arc is extinguished by a momentary power failure or deliberate disconnection of the power supply, it cannot be restarted immediately. While the arc tube is still hot, the pressure created by the still-vaporized mercury is too high to permit the formation of the glow discharge at the starting electrode. A cooling period, ordinarily on the order of about five minutes, is necessary to allow the mercury to condense on the arc-tube walls, lowering the pressure sufficiently for the process to begin again.
The prospect of being without light for a period of time in the presence of just a temporary cessation of power has meant that fluorescent lights have been used in installations where gaseous-discharge lamps would have otherwise been preferred.
Also, in a very cold environment, the possibility that an initial glow discharge at the starting electrode will not be initiated is a distinct probability. The ambient temperature where striking of an arc may normally be considered assured for most gaseous-discharge lamps is 50.degree. F. When the temperature is expected to be lower than this level, and particularly where the temperature is expected to be appreciably lower than this level, the unreliable starting performance of such lamps has meant that gaseous-discharge lamps have not been used where they would have been otherwise. Before the present invention, when gaseous-discharge lamps are used in a low temperature situation, it has been necessary to use expensive ballasts having higher internal losses than ballasts normally employed.
Among the objects and the advantages of the present invention is to overcome these two principal shortcomings by providing an emergency lighting circuit that will light in the event that the gaseous-discharge lamp with which it operates fails. Also, certain embodiments of the present invention include locating the incandescent lights of the emergency lighting circuit to raise the ambient operating temperature of the gaseous-discharge lamps, thereby aiding its ignition.
The various types of vapor lamps all possess a negative resistance characteristic, wherein the resistance within the lamp envelope decreases with an increase in current. Without some form of current-limiting device in the electric circuit, current would rise swiftly after the lamp started until lamp failure occurred. This current-limiting element is external to the gaseous-discharge envelope and, as referred to above, is called the ballast. Ballasts of complex structures using inductive and capacitive reactances have been employed in AC circuits and have increased efficiencies when compared with resistive-type ballasts. Ballasts may take the form of simple chokes, transformers, autotransformers, a combination of these or other structures. Although ballast circuits of a complex nature have been used, heretofore none has operated in the manner of the present circuits, as will be explained more fully hereinafter.
The present invention operates conjunctively not only with the gaseous-discharge lamp with which it is connected, but also with a ballast, which would be required for operation of the gaseous-discharge lamp in any event. The present invention is not restricted to the type of ballast employed and should not be characterized by itself as a ballast circuit, but rather should be thought of as normally including an emergency lighting circuit.